Aluminium and its alloys are a widely used material for automotive, structural and aerospace applications, however without suitable functional coatings many alloys suffer from environmental degradation due to corrosion. A number of processes have been developed to protect aluminium surfaces including anodizing, plating and chemical films. However, to effectively protect the aluminium surface either a thick plated or anodized film is required. Alternatively, thin films of environmentally hazardous materials such as cadmium or hexavalent chromium are required.
Anodizing is one well recognized method to protect aluminium and other light metal surfaces. Different applications of an anodized surface may utilize either a thick film, where high protection is required, or a thin film for more decorative applications. In thick film or hard anodizing an oxide surface between 25 and 150 microns thick is developed. This surface is typically sealed in a process that may include dying. Other protective coatings may be subsequently applied to this surface. Two patents U.S. Pat. Nos. 4,431,707 and 4,624,752 describe methods to further treat hard anodized surfaces so that they may be plated. Both of these methods include a chemical etch phase to create a layer to which an electrically conductive surface may be applied and plated layers electrodeposited on this surface.
Thin film anodized surfaces are typically between 0.5 and 25 microns. As with hard anodizing these surfaces are normally sealed to provide environmental protection. An advantage of thin anodized surfaces is that sufficient electrical conductivity remains between the substrate through the anodizing pores that it is possible to directly electrodeposit functional films on the anodized surface. Patents U.S. Pat. Nos. 3,915,811 and 3,943,039 describe methods to further treat anodized films and electro deposit, especially nickel coatings, on such films. These patents specify different baths and processes for the anodizing while suggesting a variety of approaches to electro-deposition to provide a functional surface. Both these patents are directed at a subset of aluminium alloys of particular importance to the automotive industry for car bumpers and typically involve electro-depositing one or more thick layers to achieve the corrosion protection and decorative aspects of these applications. More specifically these patents do not teach the approach disclosed in this application to ensure complete filling of the anodizing pores and allow thin film electro-deposited surfaces to achieve good corrosion protection and other functional properties.
Electro-deposition on aluminium is also well known and the process typically involves applying a very thin zinc layer to the surface using a zincate process followed by applying one or more plated coatings on this surface. The zincate process is inherently problematic and essential to achieve a good electrodeposited coating, thus double and triple zincate steps are often required to achieve acceptable results. In many instances, the first plated layer is a thick (40-50 microns) electro-less Ni—P coating or semi-bright electrolytic nickel to provide corrosion protection. This first layer is followed by a functional or decorative surface layer which may be a bright nickel. In one application, the surface coating is electro-deposited Zn—Ni. The Ni—P/Zn—Ni coating system has been developed to replace environmentally dangerous chromate passivated cadmium for electrical connector shells. However, the process is both expensive in time and materials and not as effective as the coating it is designed to replace.
Thin anodized films are also used as a template to produce nano-wires for sensors, such as that described in US 2009/0242416. While this patent teaches plating in the pores of an anodized surface it does not teach controlling the current to ensure complete filling of the nano-pores and achieving an interlock between the nano-wire and the pore. Nor does it teach increasing the current when the pores are filed to ensure complete coverage of the anodized film.
Consequently, there is a need in the art for a method to coat aluminium and other light metal surfaces with thin plated coatings that provides protection from corrosion and other functional attributes.